A critical aspect of the field of nonlinear optics (NLO) is focused upon the response of materials to electromagnetic fields. Interest often relates to how materials generate new electromagnetic fields with altered properties, such as frequency and phase, upon irradiation with an external electromagnetic field. Materials efficient at nonlinear photonic signal manipulation are of interest for a large number of technological applications including optical communications and computation, optical switching and limiting, data storage and retrieval, and dynamic image processing, among numerous others. (Prasad, P. R. and Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; John Wiley and Sons: New York, 1991.) One of the primary limitations encountered, however, is the availability of suitable materials with large nonlinear responses. Most molecular nonlinear optical materials are inefficient photonic modulators. Major research initiatives have, therefore, been directed both toward gaining a detailed understanding of fundamental structure-optical property relationships and the theoretical modeling, experimental synthesis and NLO property measurement of new materials. A great deal of computational and experimental work, primarily with organic systems, has begun to address these important relationships. The present invention relates to the use of new classes of compounds as nonlinear optical materials.
Molecular NLO materials have many particularly attractive properties including ultrafast response times, lower dielectric constants, significantly improved processability, facile three dimensional design capabilities, and greatly enhanced NLO responses. (Blau, W. Phys. Technol. 1987, 18, 250.) Most molecular NLO materials, for example, employ electron donating (donors) and withdrawing groups (acceptors) connected through an organic framework (bridge), although several metalloorganic systems have also been explored. (Cummings, S. D., Cheng, L.-T. and Eisenberg, R. Chem. Mater. 1997, 9,440.)
The design and optimization of new NLO materials has primarily involved addressing what chemical factors affect the molecular hyperpolarizabilities of the material. For example, the most commonly employed model for understanding the fundamental relationships between the second-order responses (xcex2) and molecular structure is the two-state model. (Oudar, J. L. J. Chem. Phys. 1977, 67, 446.) In qualitative terms, when the electric field component of a moderate strength incident electromagnetic wave interacts with a compound, a linear electronic polarization occurs within the compound due to photon-electron interactions. The incident oscillating electric field causes an oscillating dipole to be generated in the chromophore proportional to the applied field strength. At high incident field strength, however, the induced electronic polarization becomes nonlinear, ultimately leading to second, third, etc. harmonic generation. A power series expansion has been used to describe the nonlinear behavior of the induced polarization. In rigorously centrosymmetric chromophores, the second-order response is zero since only odd terms of the power series expansion are allowed. Molecular parameters which enhance a noncentrosymmetric electronic polarization in the compound, therefore, enhance its second-order response. For organic NLO materials involving electron donating and withdrawing groups (often referred to as xe2x80x9cpush-pullxe2x80x9d systems), the value of xcex2 is primarily dictated by the intramolecular charge polarization, the transfer integral and the excited state of the compound. The two-state model assumes that the large second-order response (xcex2) is due primarily to an intramolecular charge-transfer interaction between the acceptor and donor portions of the material. The overall value of xcex2 is given by the sum of an additive portion (xcex2add) and a charge transfer portion (xcex2ct). The additive portion (xcex2add) accounts for the interactions between the individual substituents and the organic framework. The dominant intramolecular electronic redistribution, or charge transfer contribution (xcex2ct), is given by:       β    CT    =                    12        ⁢                  π          2                            h        2              ⁢                  ω        max                              (                                    ω              max              2                        -                          4              ⁢                              ω                2                                              )                ⁢                  (                                    ω              max              2                        -                          ω              2                                )                      ⁢          η      ge      2        ⁢    Δ    ⁢          xe2x80x83        ⁢    μ  
(where xcfx89max is the absorption band maximum, xcfx89 is the frequency of the applied electric field, xcexcge is the transition dipole moment between the ground and lowest frequency excited state, and xcex94xcexc is the difference between the dipole moment of the ground and excited states). The two state model is a somewhat oversimplified description but it has been shown to be particularly useful in understanding the nonlinear optical properties of many molecular systems. Thus, in the xe2x80x9cpush-pullxe2x80x9d organic compounds, increasing the length of the xcfx80-conjugated pathway between the donating and withdrawing groups and increasing the donor/acceptor group strengths typically leads to an increase in the observed electronic molecular hyperpolarizabilities. Increasing the length of the xcfx80-framework, however, also usually leads to a bathochromic shift of the intramolecular charge transfer absorption, typically into the visible region, which often limits the usefulness of the these materials. The calculated second-order responses for twisted xcfx80-chromophores, however, have been recently shown to be unresponsive toward the typical strategies for increasing xcex2, such as by increasing both the length of the xcfx80-conjugation and the donor and acceptor strengths. This is primarily because the second-order responses for these twisted compounds are most dependent upon factors which effectively bring about and ultimately stabilize intramolecular charge separation. (Albert, I. D. L., Marks, T. J. and Ratner, M. A. J. Am. Chem. Soc. 1998, 120, 11174.).
The numeric values for xcex2 range over six orders of magnitude, typically from about 0.001 for very small compounds to nearly 1000 (xc3x9710xe2x88x9230 cm5 esuxe2x88x921) for the best extended xcfx80-conjugated systems. Values of 10 to 100 (at 0.65 eV) are usually considered large and between 100 and 1000 (xc3x9710xe2x88x9230 cm5 esuxe2x88x921) exceptionally large. It is important to observe, however, that the magnitude of xcex2 is rather sensitive to the frequency of the electromagnetic radiation employed and generally increases significantly with increasing excitation energy. In addition, these qualitative descriptions do not apply at near resonant frequencies. (Kanis, D. R., Ratner, M. A. and Marks, T. J. Chem. Rev. 1994, 94, 195). Table 1 gives several example molecular hyperpolarizabilities for known high xcex2 NLO systems.
Calculational methods have been used to great effect in studying the relationships between nonlinear responses and molecular architectures. Numerous methods have been extensively and effectively employed including semiempirical methods (MOPAC and ZINDO principally), density functional theory (DFT) and ab initio methods. Significant advantages in calculational speed have been particularly realized by employing semiempirical methods (such as MOPAC AM1) which apparently retain the NLO calculational accuracy obtained with higher order ab initio basis sets (such as 6-31 G* *). Where experimental data exists, exceptionally good agreement is generally obtained between the calculated and experimental values of xcex2 as illustrated in Table 1.
A key component of research in NLO systems has involved the discovery of new materials efficient at nonlinear photonic signal manipulation. As described above, the value of xcex2 for molecular NLO materials is primarily determined by the intramolecular polarization, the transfer integral and the excited states of the compound. Chemical approaches designed to optimize these parameters by modifying the molecular architectures of the chromophore should directly result in increases in the observed nonlinear response. Several approaches have been developed for organic systems which have directly resulted in some observed increases in xcex2 for new materials (vide supra). It is apparent, however, that significantly better materials are yet possible. Since most of the known NLO materials are rather inefficient photonic modulators, it can clearly been seen that, in order for significant technological advances to be made in the use of NLO materials in opto-electronic and related applications, materials with significantly better NLO responses are required. In addition, current molecular NLO materials have significant problems associated with their chemical and thermal stability, processability, and synthetic facility. This invention describes new classes of compounds with both greatly enhanced NLO properties and significantly improved chemical and physical attributes.